According to Sony customer support, it'll work backwards as well. Though, there's also reports of Sony customer support saying it won't, too.
http://www.gamepur.com/news/16099-pre-ordered-destiny-ps4-digital-version-you-will-get-ps3-version-free-after.html
I REALLY hope it works backwards, as well. My living room has the XBone which is what I purchased it for, but I'd be able to play on the 360 in my room way more often.
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I have been waiting for a response to this for some time, I had ordered the games before I was made aware of the ability to upgrade, clearly I would have bought the PS3 version if this was made available at the time. To date the PS3 (retrograde) is still not available for download. Bungie PSN need to be more clear on what the situation is as I believe we have been duped into buying PS4 versions. Can Bungie please allow retrograde downloads for those of us who have bought the PS4 version on the premise there wasn't an upgrade option. Obviouslt the net result is exactly the same!! http://www.destinythegame.com/uk/en/digital-upgrade http://www.gamepur.com/news/16099-pre-ordered-destiny-ps4-digital-version-you-will-get-ps3-version-free-after.html
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It's not available for Xbox, either. I've contacted microsoft customer support twice about it, and both times I was told there was nothing they can do(a lie), they weren't sure why a previous customer support agent said it would be available, and to try and contact the game developers.